1. Technical Field
This invention applies to gas turbine engine fuel controls in general, and to gas turbine engine fuel flow control valves in particular.
2. Background Information
Fuel control valves for high performance gas turbine powered aircraft must perform with a high degree of accuracy under a wide variety of operating conditions. A valve that meters too little or too much fuel to the combustor could cause a combustor to "blowout", or could hinder reignition within the combustor. To avoid such problems, valve design considers the difference in pressure across the fuel control valve and the mass flow rate of fluid through the valve. These two parameters are generally used to define the required performance of the fuel control valve within the aircraft flight envelope. The difference in pressure (.DELTA..sub.p) "across" the valve is by consensus defined to be the difference between the pressure of the fuel discharging from the fuel pump less component and piping head losses between the pump discharge and the control valve (P.sub.FPD) and the pressure of the fuel dispensed within the combustor(s) less component and piping head losses between the control valve and the combustor (P.sub.FC)
The mass flow rate of the fluid passing through the valve, on the other hand, may be determined by the equation: ##EQU1## where W.sub.f represents the mass flow rate of the fluid, K represents a conversion factor constant, C.sub.d represents a discharge coefficient for flow exiting the valve orifice, A.sub.v represents the cross-sectional area of the valve orifice, and .rho. represents the density of the fluid. The discharge coefficient (C.sub.d) is a coefficient that compensates for less than frictionless ideal flow through an orifice and is a function of: (1) the geometry of the orifice relative to upstream passage geometry; and (2) the Reynolds number of the fluid passing through the orifice. The Reynolds number of the fluid passing through the orifice, in turn, accounts for the velocity of the fluid within the orifice, the dimensions of the orifice, and the kinematic viscosity of the fluid. In instances where the ratio of pressures across the valve (P.sub.FPD /P.sub.FC) is no more than six (6), the discharge coefficient (C.sub.d) may be considered a constant for a particular point within the flight envelope. This is in part due to a relatively low fluid velocity through the orifice. In those instances, the mass flow rate of fluid (W.sub.f), and therefore the power setting of the engine, can be readily controlled by changing only the cross-sectional area of the valve orifice (A.sub.v).
In instances where the ratio of pressures across existing valves exceed six (6), however, the discharge coefficient (C.sub.d) often becomes unstable due to cavitation and cannot be considered a constant for a particular point within the flight envelope. Specifically, at pressure ratios greater than six, the velocity of the fluid passing through the orifice is great enough to cause cavitation which in turn prevents a consistent C.sub.d value from being empirically determined. Control of fuel flow rate through the valve in these instances must, therefore, consider at least two variables, one of which is unstable. Under those circumstances accurate fuel flow control through the valve is difficult at best.
To avoid having a ratio of pressures across the fuel control valve in excess of six (6), it is known to use a hydromechanical head regulator, which is a device designed to maintain a particular .DELTA..sub.p across a fuel control valve under all conditions. Although head regulators do provide the advantage of a constant .DELTA..sub.p across the fuel flow control valve, they also provide several distinct disadvantages. For example, head regulators used in high pressure applications tend to be of considerable size and weight, neither of which is desirable. Head regulators also add a second layer of complexity to the fuel control system; e.g. they require sensor input to operate and a flow valve to regulate a constant head across a metering valve. The sensors and the valves within the head regulator provide additional potential failure modes which are difficult, if not impossible, to diagnose. Head regulators also add significantly to the cost of most gas turbine fuel flow control systems. In short, the advantage of a constant .DELTA..sub.p across the control valve is offset by several distinct disadvantages.
What is needed, therefore, is a method for accurately controlling the mass flow rate of fuel within a gas turbine engine that accommodates high pressure differences across the valve, which does not add to the weight, cost, or complexity of the fuel flow control system.